Driving methods to produce a mixed color state for an electrophoretic display

ABSTRACT

The present invention is directed to driving methods for a color display device which can display high quality color states. The display device utilizes an electrophoretic fluid which comprises three types of pigment particles having different optical characteristics, and provides for displaying at a viewing surface not only the colors of the three types of particles but also the colors of binary mixtures thereof.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/496,604, filed Apr. 25, 2017, now U.S. Pat. No. 10,380,931, which isa continuation-in-part of U.S. patent application Ser. No. 14/507,737,filed Oct. 6, 2014, now U.S. Pat. No. 10,339,876, which claims thebenefit of U.S. Provisional Applications Nos. 61/887,821, filed Oct. 7,2013; 61/925,055, filed Jan. 8, 2014; 61/942,407, filed Feb. 20, 2014;61/979,464, filed Apr. 14, 2014; and 62/004,713, filed May 29, 2014. Thecontents of the above-identified applications and of all other UnitedStates patents and published applications referred to below areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention is directed to driving methods for color displaydevices to display high quality color states.

BACKGROUND OF THE INVENTION

In order to achieve a color display, color filters are often used. Themost common approach is to add color filters on top of black/whitesub-pixels of a pixellated display to display the red, green and bluecolors. When a red color is desired, the green and blue sub-pixels areturned to the black state so that the only color displayed is red. Whena blue color is desired, the green and red sub-pixels are turned to theblack state so that the only color displayed is blue. When a green coloris desired, the red and blue sub-pixels are turned to the black state sothat the only color displayed is green. When a black state is desired,all three-sub-pixels are turned to the black state. When a white stateis desired, the three sub-pixels are turned to red, green and blue,respectively, and as a result, a white state is seen by the viewer.

The biggest disadvantage of such a technique is that since each of thesub-pixels has a reflectance of about one third (⅓) of the desired whitestate, the white state is fairly dim. To compensate for this, a fourthsub-pixel may be added which can display only the black and whitestates, so that the white level is doubled at the expense of the red,green or blue color level (where each sub-pixel is only one fourth ofthe area of the pixel). Brighter colors can be achieved by adding lightfrom the white pixel, but this is achieved at the expense of color gamutto cause the colors to be very light and unsaturated. A similar resultcan be achieved by reducing the color saturation of the threesub-pixels. Even with these approaches, the white level is normallysubstantially less than half of that of a black and white display,rendering it an unacceptable choice for display devices, such ase-readers or displays that need well readable black-white brightness andcontrast.

SUMMARY OF THE INVENTION

A first aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which comprises a first type of pigment particles, a second typeof pigment particles and a third type of pigment particles, all of whichare dispersed in a liquid, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but a lower        zeta potential,    -   the method comprising the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, the first        driving voltage having a polarity driving the first type of        pigment particles towards the first surface, thereby causing the        pixel to display the optical characteristic of the first type of        pigment particles at the first surface;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, the second driving voltage having a polarity        driving the third type of pigment particles towards the first        surface, thereby driving the pixel towards the optical        characteristic of the third type of pigment particles at the        first surface; and    -   repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage. In oneembodiment, steps (i) and (ii) are repeated at least 4 times. In oneembodiment, the method further comprises a shaking waveform before step(i). In one embodiment, the method further comprises driving the pixelto the full optical characteristic of the first type of pigmentparticles after the shaking waveform but prior to step (i). In oneembodiment, the first period of time is 40 to 140 msec, the secondperiod of time is greater than or equal to 460 msec and steps (i) and(ii) are repeated at least seven times.

A second aspect of the present invention is directed to a driving methodfor an electrophoretic display as described above but including anadditional step, as follows: applying no driving voltage to the pixelfor a third period of time after step (ii) but before repeating steps(i) and (ii); and repeating steps (i), (ii) and (iii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage. In oneembodiment, steps (i), (ii) and (iii) are repeated at least 4 times. Inone embodiment, the method further comprises applying a shaking waveformbefore step (i). In one embodiment, the method further comprises adriving step to the full color state of the first type of pigmentparticles after the shaking waveform but prior to step (i).

A third aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which comprises a first type of pigment particles, a second typeof pigment particles and a third type of pigment particles, all of whichare dispersed in a liquid, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles a lower zeta        potential,    -   and the method has a voltage insensitive range of at least 0.7        V.

A fourth aspect of the present invention is directed to a driving methodfor an electrophoretic display as per the first aspect of the inventionbut including additional steps as follows:

-   -   (iii) after step (i) but before step (ii) applying no driving        voltage to the pixel for a third period of time;    -   (iv) after step (ii) but before repeating the steps, applying no        driving voltage to the pixel for a fourth period of time; and    -   repeating steps (i)-(iv).

In one embodiment, the first type of pigment particles may be negativelycharged and the second type of pigment particles positively charged. Inone embodiment, the amplitude of the second driving voltage is less than50% of the amplitude of the first driving voltage. In one embodiment,steps (i)-(iv) are repeated at least 3 times. In one embodiment, themethod further comprises applying a shaking waveform before step (i). Inone embodiment, the method further comprises driving the pixel to thefull color state of the first type of pigment particles after theshaking waveform but prior to step (i).

A fifth aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid is sandwiched between a common electrode and a layerof pixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,    -   the method comprising the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the second type of pigment particles to drive the        pixel towards the color state of the second type of pigment        particles at the viewing side; and    -   repeating steps (i) and (ii).

In one embodiment, the method further comprises a wait time where nodriving voltage is applied. In one embodiment, the first type of pigmentparticles is negatively charged and the second type of pigment particlesis positively charged. In one embodiment, the second period of time isat least twice as long as the first period of time. In one embodiment,steps (i) and (ii) are repeated for least three times. In oneembodiment, the method further comprises applying a shaking waveformbefore step (i). In one embodiment, the method further comprises drivingthe pixel to the full color state of the second type of pigmentparticles after the shaking waveform but prior to step (i).

A sixth aspect of the present invention is directed to a driving methodfor an electrophoretic display comprising a first surface on the viewingside, a second surface on the non-viewing side and an electrophoreticfluid which fluid is sandwiched between a common electrode and a layerof pixel electrodes and comprises a first type of pigment particles, asecond type of pigment particles and a third type of pigment particles,all of which are dispersed in a solvent or solvent mixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,    -   the method comprising the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the second type        of pigment particles to drive the pixel towards the color state        of the second type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards the color state of the first type of pigment        particles at the viewing side;    -   (iii) applying no driving voltage to the pixel for a third        period of time; and    -   repeating steps (i), (ii) and (iii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, steps (i), (ii) and (iii) are repeated at least threetimes. In one embodiment, the amplitude of the second driving voltage issame as that of the driving voltage required to drive the pixel from thecolor state of the first type of pigment particles to the color state ofthe second type of pigment particles, or vice versa. In one embodiment,the amplitude of the second driving voltage is higher than the amplitudeof the driving voltage required to drive the pixel from the color stateof the first type of pigment particles to the color stat of the secondtype of pigment particles, or vice versa. In one embodiment, the methodfurther comprises applying a shaking waveform. In one embodiment, themethod further comprises driving the pixel to the full color state ofthe first type of pigment particles after the shaking waveform but priorto step (i).

A seventh aspect of the present invention is directed to a drivingmethod for an electrophoretic display comprising a first surface on theviewing side, a second surface on the non-viewing side and anelectrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,    -   the method comprising the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the second type of        pigment particles to drive the pixel towards the color state of        the second type of pigment particles wherein the first period of        time is not sufficient to drive the pixel to the full color        state of the second type of pigment particles at the viewing        side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards a mixed state of the first and second types of        pigment particles at the viewing side; and    -   repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage. In oneembodiment, steps (i) and (ii) are repeated at least 4 times. In oneembodiment, the method further comprises applying a shaking waveformbefore step (i). In one embodiment, the method further comprises drivingthe pixel to the full color state of the first type of pigment particlesafter the shaking waveform but prior to step (i).

The fourth driving method of the present invention may be applied to apixel at a color state of the first type of pigment particles or may beapplied to a pixel at a color state not the color state of the firsttype of pigment particles.

This invention also provides driving methods to display mixtures of theoptical characteristics of two of the three particles in electrophoreticdisplay fluids as previously described. A first such “mixedcharacteristic” method comprises the following steps:

-   -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, the first        driving voltage having a polarity driving the first type of        pigment particles towards the first surface, thereby causing the        pixel to display the optical characteristic of the first type of        pigment particles at the first surface;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, the second driving voltage having a polarity        driving the third type of pigment particles towards the first        surface, thereby driving the pixel towards the optical        characteristic of the third type of pigment particles at the        first surface; and    -   (iii) applying a third driving voltage for a third period of        time, the third driving voltage having the same polarity as the        first driving voltage, and the third period of time being        shorter than the first period of time, thereby producing a        mixture of the optical characteristics of the first and third        types of particles at the viewing surface.

In this first mixed characteristic method the duration of the thirdperiod of time may be from about 20 to about 80 percent, and preferablyfrom about 20 to about 40 percent, of the duration of the first periodof time. A shaking waveform may be applied prior to step (i), and adriving voltage driving the first type of pigment particles towards thefirst surface may be applied prior to the shaking waveform.

A second “mixed characteristic” method comprises the following steps:

-   -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, the first        driving voltage having a polarity driving the second type of        pigment particles towards the first surface, thereby causing the        pixel to display the optical characteristic of the second type        of pigment particles at the first surface;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, the second driving voltage having the same        polarity as, but a lower magnitude than the first driving        voltage, thereby driving the third type of pigment particles at        the first surface, and producing a mixture of the optical        characteristics of the second and third types of particles at        the first surface.

In this second mixed characteristic method, the duration of the secondperiod of time may be from about 100 to about 150 percent of theduration of the first period of time. A shaking waveform may be appliedprior to step (i), and a driving voltage driving the first type ofpigment particles towards the first surface may be applied prior to theshaking waveform.

A third “mixed characteristic” method comprises the following steps:

-   -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, the first        driving voltage having a polarity driving the first type of        pigment particles towards the first surface, thereby causing the        pixel to display the optical characteristic of the first type of        pigment particles at the first surface;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, the second driving voltage having a polarity        driving the third type of pigment particles towards the first        surface; and        repeating steps (i) and (ii),        wherein the durations of steps (i) and (ii) and the magnitudes        of the voltages applied therein are adjusted to produce a        mixture of the optical characteristics of the third type of        particles with one of the first and second types of particles at        the first surface.

In this third mixed characteristic method, a shaking waveform may beapplied prior to step (i), and a driving voltage driving the first typeof pigment particles towards the first surface may be applied prior tothe shaking waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section through an electrophoretic displayfluid useful in the driving methods of the present invention.

FIGS. 2A, 2B and 2C are schematic cross-sections, similar to that ofFIG. 1, showing the positions of the particles in, respectively, theblack, white and red (colored) states of the display fluid.

FIG. 3 illustrates a typical waveform for driving a pixel from the whitestate of FIG. 2B to the red state of FIG. 2C.

FIG. 4 illustrates a waveform which may be used to replace the portionof the waveform of FIG. 3 in period t3 to effect the first drivingmethod of the present invention.

FIGS. 5 and 6 depict waveforms produced by modifying the waveform ofFIG. 3 with the partial waveform of FIG. 4 to effect the first drivingmethod of the present invention.

FIG. 7 illustrates a second waveform which may be used to replace theportion of the waveform of FIG. 3 in period t3 to effect the seconddriving method of the present invention.

FIGS. 8 and 9 depict waveforms produced by modifying the waveform ofFIG. 3 with the partial waveform of FIG. 7 to effect the second drivingmethod of the present invention.

FIGS. 10A and 10B illustrate the optical results produced by the thirddriving method of the present invention. FIG. 10A demonstrates therelationship of applied driving voltage vs. optical state performance(a*), based on the waveform of FIG. 3, and FIG. 10B demonstrates therelationship of applied driving voltage vs. optical state performance(a*), based on the waveform of FIG. 3 modified with the partial waveformof FIG. 4.

FIG. 11 illustrates a waveform which may be used to replace the portionof the waveform of FIG. 3 in period t3 to effect the fourth drivingmethod of the present invention.

FIGS. 12 and 13 depict waveforms produced by modifying the waveform ofFIG. 3 with the partial waveform of FIG. 11 to effect the fourth drivingmethod of the present invention.

FIG. 14 depicts a typical waveform for driving a pixel from the whitestate of FIG. 2B to the black state of FIG. 2A.

FIG. 15 illustrates a waveform which may be added at the end of thewaveform of FIG. 14 to effect the fifth driving method of the presentinvention.

FIG. 16 illustrates a complex waveform combining the waveforms of FIGS.14 and 15 to effect the fifth driving method of the present invention.

FIG. 17 depicts a typical waveform for driving a pixel to the whitestate of FIG. 2B.

FIGS. 18A and 18B illustrate two waveforms which may be used to replacethe portion of the waveform of FIG. 17 in period t17 to effect the sixthdriving method of the present invention.

FIGS. 19A and 19B depict waveforms produced by modifying the waveform ofFIG. 17 with the partial waveform of FIG. 18A or 18B respectively toeffect the sixth driving method of the present invention.

FIGS. 20A and 20B are schematic cross-sections, similar to that of FIG.1, showing the positions of the particles in, respectively, the blackand grey states of the display fluid.

FIG. 21 illustrates a typical waveform for driving a pixel to the greystate of FIG. 20B.

FIG. 22 illustrates a waveform which may be used to replace the portionof the waveform of FIG. 21 in period t23 to effect the seventh drivingmethod of the present invention.

FIG. 23 illustrates a complex waveform combining the waveforms of FIGS.21 and 22 to effect the seventh driving method of the present invention.

FIG. 24 illustrates a waveform used in the eighth driving method of thepresent invention.

FIG. 25 illustrates a complex waveform combining the waveforms of FIGS.14 and 24 to effect the eighth driving method of the present invention.

FIGS. 26A and 26B illustrate production of a gray state of a pixelbeginning from the white state thereof.

FIGS. 26C and 26D illustrate production of a gray state of a pixelbeginning from the black state thereof.

FIG. 27 illustrates a waveform useful to drive the display to a lightred state via a white state in the first mixed characteristic drivingmethod of the present invention.

FIG. 28 illustrates a waveform useful to drive the display to a dark redstate in the second mixed characteristic driving method of the presentinvention.

FIG. 29 illustrates a second waveform useful to drive the display to adark red state in the third mixed characteristic driving method of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to driving methods for color displaydevices.

The device utilizes an electrophoretic fluid as shown in FIG. 1. Thefluid comprises three types of pigment particles dispersed in a liquid,typically a dielectric solvent or solvent mixture. For ease ofillustration, the three types of pigment particles may be referred to aswhite particles (11), black particles (12) and colored particles (13).The colored particles are non-white and non-black.

However, it is understood that the scope of the invention broadlyencompasses pigment particles of any colors as long as the three typesof pigment particles have distinguishable optical characteristics.Therefore, the three types of pigment particles may also be referred toas a first type of pigment particles, a second type of pigment particlesand a third type of pigment particles.

The white particles (11) may be formed from an inorganic pigment, suchas TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ or the like.

The black particles (12) may be CI pigment black 26 or 28 or the like(e.g., manganese ferrite black spinel or copper chromite black spinel)or carbon black.

The third type of particles may be of a color such as red, green, blue,magenta, cyan or yellow. The pigments for this type of particles mayinclude, but are not limited to, CI pigment PR 254, PR122, PR149, PG36,PG58, PG7, PB15:3, PY138, PY150, PY155 or PY20. Those are commonly usedorganic pigments described in color index handbooks, “New PigmentApplication Technology” (CMC Publishing Co, Ltd, 1986) and “Printing InkTechnology” (CMC Publishing Co, Ltd, 1984). Specific examples includeClamant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV fast red D3G,Hostaperm red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS,Hostaperm Green GNX, BASF Irgazine red L 3630, Cinquasia Red L 4100 HD,and Irgazine Red L 3660 HD; Sun Chemical phthalocyanine blue,phthalocyanine green, diarylide yellow or diarylide AAOT yellow.

In addition to colors, the first, second and third types of particlesmay have other distinct optical characteristics, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The liquid in which the three types of pigment particles are dispersedmay be clear and colorless. It preferably has a low viscosity and adielectric constant in the range of about 2 to about 30, preferablyabout 2 to about 15 for high particle mobility. Examples of suitabledielectric fluids include hydrocarbons such as Isopar,decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils,paraffin oil, silicon fluids, aromatic hydrocarbons such as toluene,xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalene,halogenated solvents such as perfluorodecalin, perfluorotoluene,perfluoroxylene, dichlorobenzotrifluoride,3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene,dichlorononane or pentachlorobenzene, and perfluorinated solvents suchas FC-43, FC-70 or FC-5060 from 3M Company, St. Paul Minn., lowmolecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoroethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

A display layer utilizing the display fluid of the present invention hastwo surfaces, a first surface (16) on the viewing side and a secondsurface (17) on the opposite side of the layer of display fluid from thefirst surface (16). The second surface therefore is on the non-viewingside. The term “viewing side” refers to the side at which images areviewed.

The display fluid is sandwiched between the two surfaces. On the side ofthe first surface (16), there is a common electrode (14) which is atransparent electrode layer (e.g., ITO), spreading over the entire topof the display layer. On the side of the second surface (17), there isan electrode layer (15) which comprises a plurality of pixel electrodes(15 a). However, since, as will readily be apparent to those skilled inthe technology of electrophoretic displays, the various particles (11,12, 13) react only to the electric field applied within the layer ofdisplay fluid, other electrode arrangements may be used; for example,the common electrode could be replaced by a series of strip electrodes,or by a matrix of electrodes similar to the pixel electrodes (15 a).

The display fluid is filled in display cells. The display cells may bealigned with or not aligned with the pixel electrodes. The term “displaycell” refers a micro-container which is filled with an electrophoreticfluid. Examples of “display cells” may include the cup-like microcellsas described in U.S. Pat. No. 6,930,818 and microcapsules as describedin U.S. Pat. No. 5,930,026. The micro-containers may be of any shapes orsizes, all of which are within the scope of the present application.

An area corresponding to a pixel electrode may be referred to as a pixel(or a sub-pixel). The driving of an area corresponding to a pixelelectrode is effected by applying a voltage potential difference (orknown as a driving voltage or an electric field) between the commonelectrode and the pixel electrode.

The pixel electrodes are described in U.S. Pat. No. 7,046,228, thecontent of which is incorporated herein by reference in its entirety. Itis noted that while active matrix driving with a thin film transistor(TFT) backplane is mentioned for the layer of pixel electrodes, thescope of the present invention encompasses other types of electrodeaddressing as long as the electrodes serve the desired functions.

The space between two vertical dotted lines denotes a pixel (or asub-pixel). For brevity, when “pixel” is referred to in a drivingmethod, the term also encompasses “sub-pixels”.

Two of the three types of pigment particles carry opposite chargepolarities and the third type of pigment particles is slightly charged.The term “slightly charged” or “lower charge intensity” is intended torefer to the charge level of the particles being less than about 50%,preferably about 5% to about 30%, the charge intensity of the strongercharged particles. In one embodiment, the charge intensity may bemeasured in terms of zeta potential. In one embodiment, the zetapotential is determined by Colloidal Dynamics AcoustoSizer IIM with aCSPU-100 signal processing unit, ESA EN # Attn flow through cell(K:127). The instrument constants, such as density of the solvent usedin the sample, dielectric constant of the solvent, speed of sound in thesolvent, viscosity of the solvent, all of which at the testingtemperature (25° C.) are entered before testing. Pigment samples aredispersed in the solvent (which is usually a hydrocarbon fluid havingless than 12 carbon atoms), and diluted to between 5-10% by weight. Thesample also contains a charge control agent (Solsperse 17000, availablefrom Lubrizol Corporation, a Berkshire Hathaway company; “Solsperse” isa Registered Trade Mark), with a weight ratio of 1:10 of the chargecontrol agent to the particles. The mass of the diluted sample isdetermined and the sample is then loaded into the flow through cell fordetermination of the zeta potential.

For example, if the black particles are positively charged and the whiteparticles are negatively charged, and then the colored pigment particlesmay be slightly charged. In other words, in this example, the chargescarried by the black and the white particles are much more intense thanthe charge carried by the colored particles.

In addition, the colored particles which carries a slight charge has acharge polarity which is the same as the charge polarity carried byeither one of the other two types of the stronger charged particles.Hereinafter, it will be assumed that the colored particles (13) carry acharge of the same polarity as the second (black) particles (12).

It is noted that among the three types of pigment particles, the onetype of particles which is slightly charged preferably has a largersize.

In addition, in the context of the present application, a high drivingvoltage (V_(H1) or V_(H2)) is defined as a driving voltage which issufficient to drive a pixel from one extreme color state to anotherextreme color state. If the first and the second types of pigmentparticles are the higher charged particles, a high driving voltage then(V_(H1) or V_(H2)) refers a driving voltage which is sufficient to drivea pixel from the color state of the first type of pigment particles tothe color state of the second type of pigment particles, or vice versa.For example, a high driving voltage, V_(H1), refers to a driving voltagewhich is sufficient to drive a pixel from the color state of the firsttype of pigment particles to the color state of the second type ofpigment particles, and V_(H2) refers to a driving voltage which issufficient to drive a pixel from the color state of the second type ofpigment particles to the color state of the first type of pigmentparticles. In this scenario as described, a low driving voltage (V_(L))is defined as a driving voltage which may be sufficient to drive a pixelto the color state of the third type of pigment particles (which areless charged and may be larger in size) from the color state of thefirst type of pigment particles. For example, a low driving voltage maybe sufficient to drive to the color state of the colored particles whilethe black and white particles are not seen at the viewing side.

In general, the V_(L) is less than 50%, or preferably less than 40%, ofthe amplitude of V_(H) (e.g., V_(H1) or V_(H2)).

The following is an example illustrating a driving scheme of howdifferent color states may be displayed by an electrophoretic fluid asdescribed above.

Example 1

This example is demonstrated in FIGS. 2A-2C. The white pigment particles(21) are negatively charged while the black pigment particles (22) arepositively charged, and both types of the pigment particles are smallerthan the colored particles (23).

The colored particles (23) carry the same charge polarity as the blackparticles, but are slightly charged. As a result, the black particlesmove faster than the colored particles (23) under certain drivingvoltages.

In FIG. 2A, the applied driving voltage is +15V (i.e., V_(H1), i.e., thepixel electrode is at +15 V relative to the common electrode). In thiscase, the negative white particles (21) move to be near or at therelatively positive pixel electrode (25) and the positive blackparticles (22) and the positive colored particles (23) move to be nearor at the relatively negative common electrode (24). As a result, theblack color is seen at the viewing side. The colored particles (23) movetowards the common electrode (24) at the viewing side; however becausetheir lower charge intensity and larger size, they move slower than theblack particles.

In FIG. 2B, when a driving voltage of −15V (i.e., V_(H2)) is applied,the negative white particles (21) move to be near or at the relativelypositive common electrode (24) at the viewing side and the positiveblack particles and the positive colored particles move to be near or atthe relatively negative pixel electrode (25). As a result, the whitecolor is seen at the viewing side.

It is noted that V_(H1) and V_(H2) have opposite polarities, and havethe same amplitude or different amplitudes. In the example as shown inFIG. 2, V_(H1) is positive (the same polarity as the black particles)and V_(H2) is negative (the same polarity as the white particles)

The driving from the white color state in FIG. 2B to the colored statein FIG. 2C may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles (i.e., white), a second type of pigment particles(i.e., black) and a third type of pigment particles (i.e., colored), allof which are dispersed in a solvent or solvent mixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises driving a pixel in the electrophoretic        display from the color state of the first type of pigment        particles towards the color state of the third type of pigment        particles by applying a low driving voltage which is sufficient        to drive the third type of pigment particles to the viewing side        while leaving the first and second types of pigment particles on        the non-viewing side and the polarity of the low driving voltage        applied is the same as the polarity of the third type of pigment        particles.

In order to drive a pixel to the color state of the third type ofpigment particles, i.e., red (see FIG. 2C), the method starts from thecolor state of the first type of pigment particles, i.e., white (seeFIG. 2B).

When the color of the third type of particles is seen at the viewingside, the other two types of the particles may be mixed at thenon-viewing side (side opposite of the viewing side), resulting in anintermediate color state between the colors of the first and secondtypes of particles. If the first and second types of particles are blackand white and the third type of particles is red, then in FIG. 2C, whenthe red color is seen at the viewing side, a grey color is at thenon-viewing side.

The driving method ideally would ensure both color brightness (i.e.,preventing the black particles from being seen) and color purity (i.e.,preventing the white particles from being seen) in the scenario of FIG.2C. However, in practice, this desired result is difficult to achievefor various reasons, including particle size distribution and particlecharge distribution.

One solution to this is the use of a shaking waveform prior to drivingfrom the color state of the first type of pigment particles (i.e.,white) to the color state of the third type of pigment particles (i.e.,red). The shaking waveform consists of repeating a pair of oppositedriving pulses for many cycles. For example, the shaking waveform mayconsist of a +15V pulse for 20 msec and a −15V pulse for 20 msec andsuch a pair of pulses is repeated for 50 times. The total time of such ashaking waveform would be 2000 msec. The notation, “msec”, stands formillisecond.

The shaking waveform may be applied to a pixel regardless of the opticalstate (black, white or red) prior to a driving voltage being applied.After the shaking waveform is applied, the optical state would not be apure white, pure black or pure red. Instead, the color state would befrom a mixture of the three types of pigment particles.

For the method as described above, a shaking waveform is applied priorto the pixel being driven to the color state of the first type ofpigment particles (i.e., white). With this added shaking waveform, eventhough the white state is measurably the same as that without theshaking waveform, the color state of the third type of pigment particles(i.e., red) would be significantly better than that without the shakingwaveform, on both color brightness and color purity. This is anindication of better separation of the white particles from the redparticles as well as better separation of the black particles from thered particles.

Each of the driving pulses in the shaking waveform is applied for notexceeding half of the driving time required for driving from the fullblack state to the full white state, or vice versa. For example, if ittakes 300 msec to drive a pixel from a full black state to a full whitestate, or vice versa, the shaking waveform may consist of positive andnegative pulses, each applied for not more than 150 msec. In practice,it is preferred that the shaking waveform pulses are shorter.

It is noted that in all of the drawings throughout this application, theshaking waveform is truncated (i.e., the number of pulses is fewer thanthe actual number).

The waveform used to drive the display to the colored (red) state ofFIG. 2C is shown in FIG. 3. In this waveform, a high negative drivingvoltage (V_(H2), e.g., −15V) is applied for a period of t2, to drive apixel towards a white state after a shaking waveform. From the whitestate, the pixel may be driven towards the colored state (i.e., red) byapplying a low positive voltage (V_(L), e.g., +5V) for a period of t3(that is, driving the pixel from FIG. 2B to FIG. 2C).

The driving period “t2” is a time period sufficient to drive a pixel tothe white state when V_(H2) is applied and the driving period “t3” is atime period sufficient to drive the pixel to the red state from thewhite state when V_(L) is applied. A driving voltage is preferablyapplied for a period of t1 before the shaking waveform to ensure DCbalance. The term “DC balance”, throughout this application, is intendedto mean that the driving voltages applied to a pixel is substantiallyzero when integrated over a period of time (e.g., the period of anentire waveform).

The First Driving Method:

A waveform useful in the first driving method of the present inventionis illustrated in FIG. 4; this waveform may be used to replace thedriving period of t3 in FIG. 3.

In an initial step, a high negative driving voltage (V_(H2), e.g., −15V)is applied, which is followed by a positive driving voltage (+V′) todrive a pixel towards the red state. The amplitude of the +V′ is lessthan 50% of the amplitude of V_(H) (e.g., V_(H1) or V_(H2)).

In this driving waveform, a high negative driving voltage (V_(H2)) isapplied for a period of t4 to push the white particles towards theviewing side, which is then followed by applying a positive drivingvoltage of +V′ for a period of t5, which pulls the white particles downand pushes the red particles towards the viewing side.

In one embodiment, t4 may be in the range of 20-400 msec and t5 may be≥200 msec.

The waveform of FIG. 4 is repeated for at least 4 cycles (N≥4),preferably at least 8 cycles. The red color becomes more intense aftereach driving cycle.

The driving method of FIG. 4 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side; and    -   repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage.

As stated, the driving waveform as shown in FIG. 4 may be used toreplace the driving period of t3 in FIG. 3, and FIG. 5 illustrates thecombined waveform after this replacement. In other words, the drivingsequence may be: shaking waveform, followed by driving towards the whitestate for a period of t2 and then applying the waveform of FIG. 4.

In another embodiment, the step of driving to the white state for aperiod of t2 may be eliminated and in this case, a shaking waveform isapplied immediately before applying the waveform of FIG. 4 (see FIG. 6).

In one embodiment, the driving sequence of FIG. 5 or FIG. 6 is DCbalanced.

The Second Driving Method:

A waveform useful in the second driving method of the present inventionis illustrated in FIG. 7. This waveform is an alternative to the drivingwaveform of FIG. 4, and may also be used to replace the driving periodof t3 in FIG. 3.

In this alternative waveform, there is a wait time “t6” added after thered-going pulse in period t5 and before the white-going pulse in periodt4 and the red-going-pulse in period t5 are repeated. During the waittime, no driving voltage is applied. The entire waveform of FIG. 7 isalso repeated for multiple cycles (for example, N≥4).

The waveform of FIG. 7 is designed to release the charge imbalancestored in the dielectric layers in an electrophoretic display device,especially when the resistance of the dielectric layers is high, forexample, at a low temperature.

In the context of the present application, the term “low temperature”refers to a temperature below about 10° C.

The wait time presumably can dissipate the unwanted charge stored in thedielectric layers and cause the short pulse (“t4”) for driving a pixeltowards the white state and the longer pulse (“t5”) for driving thepixel towards the red state to be more efficient. As a result, thisalternative driving method will bring a better separation of the lowcharged pigment particles from the higher charged ones. The wait time(“t6”) can be in a range of 5-5,000 msec, depending on the resistance ofthe dielectric layers.

This driving method of FIG. 7 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid which fluid is sandwichedbetween a common electrode and a layer of pixel electrodes and comprisesa first type of pigment particles, a second type of pigment particlesand a third type of pigment particles, all of which are dispersed in asolvent or solvent mixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iii) applying no driving voltage to the pixel for a third        period of time; and        repeating steps (i), (ii) and (iii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, the amplitude of the second driving voltage is lessthan 50% of the amplitude of the first driving voltage.

As stated, the driving waveform shown in FIG. 7 may also be used toreplace the driving period of t3 in FIG. 3 (see FIG. 8). In other words,the driving sequence may be: shaking waveform, followed by drivingtowards the white state for a period of t2 and then applying thewaveform of FIG. 7.

In another embodiment, the step of driving to the white state for aperiod of t2 may be eliminated and in this case, a shaking waveform isapplied before applying the waveform of FIG. 7 (see FIG. 9).

In another embodiment, the driving sequence of FIG. 8 or FIG. 9 is DCbalanced.

It should be noted that the lengths of any of the driving periodsreferred to in this application may be temperature dependent.

The Third Driving Method:

FIG. 10A demonstrates the relationship between applied driving voltage(V′) and the optical performance, based on the waveform of FIG. 3. Asshown, the positive driving voltage V′ applied may impact on the redstate performance of a color display device described above. The redstate performance of the display device is expressed as a* value,utilizing the L*a*b* color system.

The maximum a* in FIG. 10A appears at the applied driving voltage V′, inFIG. 3, being about 3.8V. However, if a change of ±0.5V is made to theapplied driving voltage, the resulting a* value would be about 37 whichis roughly 90% of the maximum a*, thus still acceptable. This tolerancecan be beneficial to accommodate changing of the driving voltages causedby, for example, variation in the electronic components of a displaydevice, the drop of battery voltage over time, batch variation of theTFT backplanes, batch variation of the display devices or temperatureand humidity fluctuations.

Based on the data given in FIG. 10A, a study was performed to find arange of driving voltages V′ that can drive to the red state with anover 90% of the maximum a* value. In other words, when any of thedriving voltages in the range is applied, the optical performance is notsignificantly affected. Therefore, the range may be referred to as“voltage-insensitive” range. The wider the “voltage insensitive” range,the more tolerant the driving method is to batch variations andenvironmental changes.

In FIG. 4, there are three parameters which need to be considered forthis study, t4, t5 and N. The effects of the three parameters on thevoltage-insensitive range are interactive and non-linear.

Following the model of FIG. 10A, one can find the optimum value sets forthe three parameters to achieve the widest voltage-insensitive range forthe waveform of FIG. 4. The results are summarized in FIG. 10B.

When t4 is between 40-140 msec, t5 is greater than or equal to 460 msecand N is greater than or equal to 7, the voltage-insensitive range(i.e., 3.7V to 6.5V) based on FIG. 10B is twice the width of thevoltage-insensitive range (i.e., 3.3V-4.7V) based on FIG. 10A.

The optimized parameters discussed above are also applicable to any ofthe driving methods of the present invention.

The third driving method therefore may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrode and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        and the method has a voltage insensitive range of at least 0.7V.

In such a method, when a driving voltage within such a range is applied,the optical quality of a color state achieved is at least 90% of themaximum acceptable “a*” value.

It is also noted that the data shown in FIGS. 10A and 10B are collectedat ambient temperature.

The Fourth Driving Method:

A waveform useful in the fourth driving method of the present inventionis illustrated in FIG. 11. This driving waveform may be used to replacethe driving period of t3 in FIG. 3.

In an initial step, a high negative driving voltage (V_(H2), e.g., −15V)is applied to a pixel for a period of t7 (cf. the corresponding pulse inperiod t4 in FIG. 4). This pulse is followed by a wait time of t8,during which no voltage is applied. After the wait time, a positivedriving voltage (V′, e.g., less than 50% of V_(H1) or V_(H2)) is appliedto the pixel for a period of t9 (cf. the corresponding pulse in periodt5 in FIG. 4)′ After the pulse in t9, but before the various steps ofthe waveform are repeated, there is a second wait time of t10, duringwhich no voltage is applied. The waveform of FIG. 11 is repeated Ntimes. The term, “wait time”, as described above, refers to a period oftime in which no driving voltage is applied.

This driving method not only is particularly effective at a lowtemperature, it can also provide a display device better tolerance ofstructural variations caused during manufacture of the display device.Therefore its usefulness is not limited to low temperature driving.

In the waveform of FIG. 11, the first wait time t8 is very short whilethe second wait time t10 is longer. The period of t7 is also shorterthan the period of t9. For example, t7 may be in the range of 20-200msec; t8 may be less than 100 msec; t9 may be in the range of 100-200msec; and t10 may be less than 1000 msec.

FIG. 12 shows the waveform produced by inserting the waveform of FIG. 11in place of the period t3 in FIG. 3. In FIG. 3, a white state isdisplayed during the period t2. As a general rule, the better the whitestate in this period, the better the red state that will be displayed atthe end of the waveform.

In the shaking waveform, the positive/negative pulse pair is preferablyrepeated 50-1500 times and each pulse is preferably applied for 10 msec.

In one embodiment, the step of driving to the white state for a periodof t2 may be eliminated and in this case, a shaking waveform is appliedbefore applying the waveform of FIG. 11 (see FIG. 13).

The fourth driving method of FIG. 11 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying no driving voltage to the pixel for a second        period of time;    -   (iii) applying a second driving voltage to the pixel for a third        period of time, wherein the second driving voltage has same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iv) applying no driving voltage to the pixel for a fourth        period of time; and        repeating steps (i)-(iv).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, steps (i)-(iv) are repeated at least 3 times.

In one embodiment, the second driving voltage is less than 50% of thedriving voltage sufficient to drive a pixel from the color state of thefirst type of pigment particles to the color state of the second type ofpigment particles, or vice versa.

In another embodiment, the driving sequence of FIG. 12 or FIG. 13 is DCbalanced.

The Fifth Driving Method:

As shown in FIG. 2A, because the black particles and the red particlescarry the same charge polarity, they tend to move in the same direction.Even though the black particles move faster than the red particles undercertain driving voltages because of their higher charge and possiblyalso smaller size, some of the red particles may still be driven to theviewing side with the black particles, to cause the quality of the blackstate to degrade.

FIG. 14 depicts a typical waveform for driving a pixel towards the blackstate. A shaking waveform (explained above) is included to ensure colorbrightness and purity. As shown, a high positive driving voltage e.g.,(V_(H1), +15V) is applied for a period of t12 to drive a pixel towards ablack state after the shaking waveform. A driving voltage is applied fora period of t11 before the shaking waveform to ensure DC balance.

FIG. 15 illustrates a waveform which may be added at the end of thewaveform of FIG. 14, for driving a pixel towards the black state. Thecombined waveform can further provide better separation of the blackparticles from the red particles, rendering the black state moresaturated, with less red tinting.

In FIG. 15, a short pulse “t13” of V_(H2) (negative) is applied,followed by a longer pulse “t14” of V_(H1) (positive) and a wait time(0V) of t15. Such a sequence is applied for at least once, preferably atleast 3 times (i.e., N is ≥3) and more preferably at least five to seventimes.

The pulse “t14” is usually at least twice the length of the pulse “t13”.

The short pulse “t13” of V_(H2) will push the black and red particlestowards the pixel electrode and the longer pulse “t14” of V_(H1) willpush them to the common electrode side (i.e., the viewing side). Sincethe speed of the two types of pigment particles are not the same underthe same driving voltages, this asymmetrical driving sequence willbenefit the black particles more than the red particles. As a result,the black particles can be better separated from the red particles.

The wait time “t15” is optional, depending on the dielectric layers inthe display device. It is common that at a lower temperature, theresistance of the dielectric layers is more pronounced and, in thiscase, a wait time may be needed to release the charge trapped in thedielectric layers.

The fifth driving method of FIG. 15 may be summarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the second type of pigment particles to drive the        pixel towards the color state of the second type of pigment        particles at the viewing side;    -   (iii) optionally applying no driving voltage to the pixel for a        third period of time; and        repeating steps (i), (ii) and (iii) if present.

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

FIG. 16 shows the complex waveform combining the waveform of FIG. 14 andthe waveform of FIG. 15. However it is also noted that, depending on theparticle speed and the cycle number (N) of the sequence, “t12” may beshortened. In other words, at the end of “t12”, the pixel does not haveto be at the full black state. Instead, the waveform of FIG. 15 couldstart at any state from black to white, including grey, provided thatthe number (N) in the sequence is sufficient to drive the pixel to theblack state at the end.

The method as described in FIGS. 14-16 may also be utilized to drive apixel to the black state at a low temperature. In this case, the periodt14 should be longer than t13 and the wait time t15 should be at least50 msec.

In one embodiment, the driving sequence of FIG. 16 is DC balanced.

The Sixth Driving Method:

FIG. 17 depicts a typical waveform for driving a pixel to a white state.A shaking waveform (explained above) is included to ensure colorbrightness and purity. A driving voltage of V_(H2) is applied for aperiod of t17 after the shaking waveform. A driving voltage of V_(H1) isapplied for a period of t16 before the shaking waveform to ensure DCbalance.

FIGS. 18A and 18B show waveforms which may be used to replace the pulset17 in the waveform of FIG. 17.

This driving method is particularly suitable for low temperaturedriving, although it is not limited to low temperature driving.

In FIG. 18A, a short pulse “t18” of V_(H1) (positive) is applied,followed by a longer pulse “t19” of V_(H2) (negative) and a wait time(0V) of t20. As shown in FIG. 18B, the amplitude of the negative drivingvoltage (V″) applied during t19 may be higher than that of V_(H2) (e.g.,−30V instead of −15V).

Such a sequence is applied for at least once, preferably at least 3times (i.e., N is ≥3 in FIGS. 18A and 18B, and more preferably at leastfive to seven times.

t19 should be longer than t18. For example, t18 may be in the range of20-200 msec and t19 may be less than 1000 msec. The wait time t20 shouldbe at least 50 msec.

The sixth driving method as shown in FIGS. 18A and 18B may be summarizedas follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the second type        of pigment particles to drive the pixel towards the color state        of the second type of pigment particles at the viewing side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, wherein the second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards the color state of the first type of pigment        particles at the viewing side;    -   (iii) applying no driving voltage to the pixel for a third        period of time; and        repeating steps (i) and (ii).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment as shown in FIG. 18A, the second voltage is thedriving voltage required to drive a pixel from the color state of thefirst type of pigment particles towards the color state of the secondtype of pigment particles, or vice versa.

In another embodiment as shown in FIG. 18B, the second voltage has aamplitude higher than that of the driving voltage required to drive apixel from the color state of the first type of pigment particlestowards the color state of the second type of pigment particles, or viceversa.

FIGS. 19A and 19B show complex waveforms combining the waveform of FIG.17 with the waveform of FIG. 18A or 18B, respectively.

In the shaking waveform, the positive/negative pulse pair is preferablyrepeated 50-1500 times and each pulse is preferably applied for 10 msec.

In one embodiment, the driving sequence of FIG. 19A or FIG. 19B is DCbalanced.

The Seventh Driving Method:

The seventh driving method of the present invention drives a pixeltowards an intermediate color state (e.g., grey).

FIGS. 20A and 20B illustrates the particle movements involved. As shown,a pixel in the black state (see FIG. 20A) is driven towards a grey statewhen a low negative driving voltage (V_(L), e.g., −5V) is applied. Inthe process, the low driving voltage pushes the red particles towardsthe pixel electrode and a mixture of black and white particles is seenat the viewing side.

The waveform used for this driving method is shown in FIG. 21. A highpositive driving voltage (V_(H1), e.g., +15V) is applied for a timeperiod of t22 to drive a pixel towards a black state, after a shakingwaveform. From the black state, the pixel may be driven towards the greystate by applying a low negative driving voltage (V_(L), e.g., −5V) fora period of t23, that is, driven from FIG. 20A to FIG. 20B.

The driving period t22 is a time period sufficient to drive a pixel tothe black state when V_(H1) is applied, and t23 is a time periodsufficient to drive the pixel to the grey state from the black statewhen V_(L) is applied. Prior to the shaking waveform, a pulse of V_(H2)is preferably applied for a period of t21 to ensure DC balance.

FIG. 22 illustrates to a driving waveform which may be used to replacethe pulse t23 in FIG. 21. In an initial step, a high positive drivingvoltage (V_(H1), e.g., +15V) is applied for a short period of t24 topush the black particles towards the viewing side, but t24 is notsufficient to drive the pixel to the full black state, and is followedby applying a low negative driving voltage (V_(L), e.g., −5V) for aperiod of t25 to drive the pixel towards a grey state. The amplitude ofV_(L) is less than 50% of V_(H) (e.g., V_(H1) or V_(H2)).

The waveform of FIG. 22 is repeated for at least 4 cycles (N≥4),preferably at least 8 cycles.

The time period, t24 is less than about 100 msec and t25 is usuallygreater than 100 msec, both at ambient temperature.

The seventh driving method as shown in FIG. 22 may be summarized asfollows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        which method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, which first        driving voltage has the same polarity as the second type of        pigment particles to drive the pixel towards the color state of        the second type of pigment particles wherein the first period of        time is not sufficient to drive the pixel to the full color        state of the second type of pigment particles at the viewing        side;    -   (ii) applying a second driving voltage to the pixel for a second        period of time, which second driving voltage has the same        polarity as the first type of pigment particles to drive the        pixel towards a mixed state of the first and second types of        pigment particles at the viewing side; and        repeating steps (i) and (ii).

As stated above, the second driving voltage is about 50% of the firstdriving voltage, in this method.

FIG. 23 shows the complex waveform combining the waveform of FIG. 21 andthe waveform of FIG. 22, in which the driving period t23 in FIG. 21 isreplaced with the waveform of FIG. 22. The complex waveform consists offour phases. The first phase is the DC balance phase (t21); the secondphase is a shaking step; and the third phase is driving a pixel to theblack state (t22). The waveform used in the third phase can be anywaveform which drives the pixel to a good black state. The fourth phaseconsists of a high positive driving voltage for a short period of timet24, followed by a low negative driving voltage for a longer period oftime t25. The fourth phase, as stated, is repeated several times.

It is noted that in FIG. 23, t22 may be optional.

It is possible to modulate the grey state to be brighter or darker bychanging the low negative voltage (V_(L)). In other words, the waveformsequence and shape may remain the same; but the amplitude of V_(L)varies (e.g. −4V, −5V, −6V or −7V) to cause different grey levels to bedisplayed. This feature could potentially reduce the required space forthe look-up tables in the driving circuit, consequently lowering thecost. The driving method as illustrated can produce a high quality of anintermediate state (of the first type of pigment particles and thesecond type of pigment particles) with very little color interferencefrom the third type of pigment particles.

In one embodiment, the driving sequence of FIG. 23 is DC balanced.

The Eighth Driving Method:

FIG. 24 illustrates a waveform used in the eighth driving method of thepresent invention. This waveform is intended to be applied to a pixelwhich is not in a white state (i.e., the color state of the first typeof pigment particles).

In an initial step, a high negative driving voltage (V_(H2), e.g., −15V)is applied for a period of t26, which is followed by a wait time of t27.After the wait time, a positive driving voltage (V′, e.g., less than 50%of V_(H1) or V_(H2)) is applied for a period of t28, which is followedby a second wait time of t29. The waveform of FIG. 24 is repeated Ntimes. The term, “wait time”, as described above, refers to a period oftime in which no driving voltage is applied.

This driving method is particularly effective at a low temperature, andit may also shorten the overall driving time to the red state.

It is noted that the time period t26 is rather short, usually in therange of about 50% of the time required to drive from a full black stateto a full white state and therefore it is not sufficient to drive apixel to a full white color state. The time period t27 may be less than100 msec; the time period t28 may range of 100-200 msec; and the timeperiod t29 may be less than 1000 msec.

The waveform of FIG. 24 is similar to that of FIG. 11, except that thewaveform of FIG. 11 is to be applied to a pixel which is in a whitestate (i.e., the color of the first type of pigment particles) whereasthe waveform of FIG. 24 is intended to be applied to a pixel which isnot in a white state.

FIG. 25 is an example wherein the waveform of FIG. 24 is applied to apixel which is at a black state (i.e., the color state of the secondtype of pigment particles).

In the shaking waveform, the positive/negative pulse pair is preferablyrepeated 50-1500 times and each pulse is preferably applied for 10 msec.

The eighth driving method of FIG. 24, like that of FIG. 11, may besummarized as follows:

A driving method for an electrophoretic display comprising a firstsurface on the viewing side, a second surface on the non-viewing sideand an electrophoretic fluid which fluid is sandwiched between a commonelectrode and a layer of pixel electrodes and comprises a first type ofpigment particles, a second type of pigment particles and a third typeof pigment particles, all of which are dispersed in a solvent or solventmixture, wherein

-   -   a) the three types of pigment particles have optical        characteristics differing from one another;    -   b) the first type of pigment particles and the second type of        pigment particles carry opposite charge polarities; and    -   c) the third type of pigment particles has the same charge        polarity as the second type of pigment particles but at a lower        intensity,        the method comprises the following steps:    -   (i) applying a first driving voltage to a pixel in the        electrophoretic display for a first period of time, wherein the        first driving voltage has the same polarity as the first type of        pigment particles to drive the pixel towards the color state of        the first type of pigment particles at the viewing side;    -   (ii) applying no driving voltage to the pixel for a second        period of time;    -   (iii) applying a second driving voltage to the pixel for a third        period of time, wherein the second driving voltage has same        polarity as the third type of pigment particles to drive the        pixel towards the color state of the third type of pigment        particles at the viewing side;    -   (iv) applying no driving voltage to the pixel for a fourth        period of time; and        repeating steps (i)-(iv).

In one embodiment, the first type of pigment particles is negativelycharged and the second type of pigment particles is positively charged.

In one embodiment, steps (i)-(iv) are repeated at least 3 times.

In one embodiment, the second driving voltage is less than 50% of thedriving voltage sufficient to drive a pixel from the color state of thefirst type of pigment particles to the color state of the second type ofpigment particles, or vice versa.

In one embodiment, the driving sequence of FIG. 25 is DC balanced.

Production of Intermediate Colors:

It is advantageous for the driving methods of the present invention tobe capable of displaying intermediate colors (i.e., mixtures of thecolors of two particles) in addition to the colors of single particles.In many cases, the displays in which the present methods are to be usedwill be required to display gray scale images which will require arealmodulation of the display. Such areal modulation increases the number ofcolors which can be displayed, but at the expense of decreasing theresolution of the display, since a number of pixels of the display aresubjected to areal modulation to form one gray scale “super pixel”.Providing each pixel of the display with the ability to displayintermediate colors, and increasing the number of intermediate colorswhich each pixel can display reduces the number of pixels which must beused in each super pixel, and hence increases the resolution of the grayscale display.

One method for the production of an intermediate gray color (i.e., amixture of the colors of the black and white particles) has already beendiscussed above with reference to FIGS. 20A and 20B. A gray color mayalso be produced by first driving a pixel to either the black or whitestate (FIG. 2A or 2B respectively) and then applying the high drivingvoltage of ±15 V to drive the pixel towards the white or black staterespectively, but terminating this driving voltage before the white orblack state is reached, thus producing a gray state. However, it shouldbe noted that, in the three particle systems used in the presentmethods, it is advantageous to produce a gray state using this methodstarting from the white state rather than from the black state, forreasons which will be explained with reference to FIGS. 26A-26D.

FIGS. 26A and 26B illustrate the production of a gray state startingfrom a white state. FIG. 26A (which is identical in substance to FIG.2B) illustrates the production of a white state by application of a highnegative driving voltage (−15V, V_(H2)), which drives the whiteparticles 21 to the viewing side and the black particles 22 and redparticles 23 towards the pixel electrode. From the white state of FIG.26A, a brief driving pulse of a high positive voltage (+15V, V_(H1))drives the white particles towards the pixel electrode and the black andred particles towards the viewing side. The brief driving pulse isterminated at a time when the white and black particles are intermixedadjacent the viewing side. Since the red particles have a lowerelectrophoretic mobility than the black particles, the red particlesmove more slowly away from the pixel electrode, and thus in the graystate are screened from a viewer by the black and white particles, whichlie between the red particles and the viewing surface. Accordingly, FIG.26B presents a “clean” gray color consisting only of a mixture of thecolors of the black and white particles and free from contamination bythe color of the red particles.

In contrast, FIGS. 26C and 26D illustrate the production of a gray statestarting from a black state. FIG. 26C (which is identical in substanceto FIG. 2A) illustrates the production of a black state by applicationof a high positive driving voltage (+15V, V_(H1)), which drives theblack particles 22 and red particles 23 towards the viewing side, andthe white particles 21 adjacent the pixel electrode. From the blackstate of FIG. 26C, a brief driving pulse of a negative voltage (−15V,V_(H2)) drives the white particles towards the viewing side and theblack and red particles towards the pixel electrode. The brief drivingpulse is terminated at a time when the white and black particles areintermixed adjacent the viewing side. However, since the red particleshave a lower electrophoretic mobility than the black particles, the redparticles move more slowly away from the viewing side, and thus in thegray state are admixed with the black and white particles; indeed, theremay be a tendency for the red particles to lie closer to the viewingside than the black particles. Accordingly, FIG. 26D presents a “dirty”gray color in which the mixture of the colors of the black and whiteparticles is significantly contaminated by the color of the redparticles.

As already noted, a gray color state of a pixel may be produced startingfrom either a black color state or a white color state. Similarly, alight red color state (a mixture of the colors of the white and redparticles) may be produced starting from either a red color state or awhite color state. In the former case, one first drives to a full redcolor state (see FIG. 2C) and then applies a high negative drivingvoltage (−15V, V_(H2)) for a brief period of time, insufficient to reachthe white color state of FIG. 2B. The high negative driving voltagecauses the white particles 21 to move rapidly towards the viewing side,the black particles 22 to move rapidly towards the pixel electrode, andthe red particles 23 to move more slowly towards the pixel electrode.The driving voltage is terminated while the white and red particles areintermingled, thus leaving a light red color visible at the viewingside. The black particles lie near the pixel electrode and thus arescreened from a viewer by the white and red particles. In the lattercase, one first drives to the full white state (see FIG. 2B) and appliesa low positive driving voltage (+5V, V_(L)) for a period of timeinsufficient to reach the red state of FIG. 2C. The low positive drivingvoltage causes the white particles 21 to move towards the pixelelectrode and the red particles to move towards the viewing side, thusagain producing an admixture of the red and white particles and thedisplay of a light red color. Instead of using a continuous low positivedriving voltage, the transition from a white state to a light red statemay be achieved using a push-pull waveform such as that illustrated inFIG. 5, 6, 8 or 9.

It has been found empirically that the light red state produced from ared state is much less uniform that than produced from a white state.Although the reasons for this difference in uniformity are not entirelyunderstood, it is believed to be related to variations of the positionsof the various particles within microcapsules (if present) andvariations in the electrophoretic mobilities of individual particles,and of the various parts of the electrophoretic display. It also appearsthat the low driving voltage used in the drive from the red color stateis more affected by variations in power supplies than the higher drivingvoltage.

FIG. 27 illustrates a waveform used to drive the display to a light redstate via a white state. In the waveform of FIG. 27, a high negativedriving voltage (V_(H2), e.g., −15V) is applied for a period of t31 todrive the pixel towards a white state. From the white state, the pixelis driven towards a red state by applying a low positive voltage (V_(L),e.g., +5V) for a period of t32, thus driving the pixel from the state ofFIG. 2B to that of FIG. 2C. Finally, the pixel is driven from the redstate to the light red state by applying a high negative driving voltage(V_(H2), e.g., −15V) for a period t33, which is shorter than the periodt31 and insufficient to drive the pixel to a full white state. A shakingwaveform is desirably applied before the white-going pulse in periodt31, and a negative driving voltage (for example, V_(H2), e.g., −15V) ispreferably applied for a period of t30 prior to the shaking waveform toensure DC balance. It will be seen that the waveform of FIG. 27 isessentially that of FIG. 3 but with the addition of the white-goingpulse in period t33. The exact shade of light red achieved can be variedby adjusting the duration of period t33, which will typically be in therange of about 20-300 msec, usually 20-100 msec. The duration of periodt33 will typically be from about 10 to about 60 percent of the durationof period t31.

Achieving a dark red color state (i.e., a mixture of the colors of theblack and red particles) is much more difficult than achieving the lightred color state because the black and red particles carry charges of thesame polarity, and hence tend to react to applied electric fields insimilar ways. For example, if one first drives a pixel to the red stateof FIG. 2C and then attempts to produce an admixture of the red andblack particles by applying the high positive drive voltage (+15V,V_(H1)) used to drive the pixel to the black state of FIG. 2A, the redparticles, which are already adjacent the front electrode as shown inFIG. 2C will remain adjacent this front electrode and will not moveaside to accommodate the arriving black particles. The result is thateven after the high positive drive voltage has been applied for a longperiod, even longer than the period required to drive the pixel from thewhite state of FIG. 2B to the black state of FIG. 2A, the resultant“dark red” state will in fact be only slightly darker than the previousred state.

It has been found that there are two methods to achieve a satisfactorydark red state. The first method uses a waveform as illustrated in FIG.28, and essentially starts from a dark gray color state. As shown inthat Figure, this waveform first applies a high positive drive voltage(+15V, V_(H1)) to drive the pixel to a dark gray state (not a full blackstate). This high positive drive voltage is followed by a low positivedrive voltage (V_(L), e.g., +5V) for a period of t36, which willnormally be significantly longer than t35, to drive the pixel to a darkred state. For reasons explained about, the high positive drive pulse inperiod t35 may optionally be preceded by a shaking waveform and/or ahigh negative drive voltage pulse (V_(H2), e.g., −15V) for a period t34.The duration of t36 can vary widely but may typically be about 300-2000msec, and more usually 500-1000 msec; the darkness of the dark red colorproduced can be varied by varying the duration of t36, with longerdurations tending to increase the redness of the color produced.

The second method of achieving a satisfactory dark red state uses awaveform as illustrated in FIG. 29, which is identical in substance toFIG. 5, although for reasons discussed below the durations of thevarious drive pulses shown in FIG. 29 will vary from those in FIG. 5. Itwill be recalled from the discussion of FIG. 5 above that the main partof the relevant waveform comprises red-going pulses of a low positivedrive voltage (V_(L), e.g., +5V), indicated as of duration t39 in FIG.29, alternating with white-going pulses of a high negative drive voltage(V_(H2), e.g., −15V) indicated as of duration t40 in FIG. 29. Thissequence of alternating pulses may be preceded by any one or more of (a)a white-going pulse of a high negative drive voltage (V_(H2), e.g.,−15V) of duration t37, intended for DC balance; (b) a shaking waveform;and (c) a white-going pulse of a high negative drive voltage (V_(H2),e.g., −15V) of duration t38, which may or may not differ from theduration t40 of the later white-going pulses already mentioned.

The waveform of FIG. 5 was described above as producing a pure red colorstate. However, it has been found empirically that, by adjusting thedurations t39 and t40 in FIG. 29, and/or by adjusting the drive voltagesV′ and V_(H2) applied during these periods, this type of waveform cangenerate not only a pure red color state but also dark red and light redcolor states. If the magnitude of V′ is increased, the red color becomesdarker, whereas if the magnitude of V′ is decreased, the red colorbecomes lighter. Similarly, if the duration of t40 is increased relativeto t39, a lighter red color will be produced, whereas if the duration oft39 is increased relative to t40, a darker red color will be produced.Obviously, combinations of changes in both driving voltage and durationmay be used. The durations of t39 and t40 can vary over a wide range;for example at 25° C., t40 can vary from 60 down to 20 msec while t39may vary from 300 up to 600 msec. Even wider ranges may be desirable atlow temperatures such as 0° C.; for example, at this temperature t40 maybe 60 msec and t39 3000 msec.

Example 2

An electrophoretic medium substantially as described above withreference to FIG. 1 was prepared by mixing 30 percent by weightpolymer-coated titania particles (white), 8 percent by weightpolymer-coated mixed metal oxide particles (black) and 7 percent byweight of red pigment particles in an isoparaffin solvent and adding acharge control agent (Solsperse 19000). The white particles borenegative charges, while both the black and the red particles borepositive charges, but the red particles had a lower charge density thatthe black particles. The resultant electrophoretic medium was loadedinto a standard test cell provided with an essentially transparent frontelectrode and driven to its white, black, red and gray states asdescribed above with reference to FIGS. 2A, 2B, 2C and 26B respectively.The L*, a* and b* values for all four colored states were measured usingstandard techniques, and the results were as follows:

TABLE 1 Color L* a* b* White 60.2 −1.0 −1.4 Black 12.6 7.7 −0.8 Red 27.037.9 17.6 Gray 38.4 −1.0 −4.0

The reflectance, Y, of the gray state was 10.3 percent. It will be seenfrom these results that the experimental medium of the present inventionwas capable of displaying good white, black and red states, and was alsocapable of displaying a gray state.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, materials, compositions, processes, process stepor steps, to the objective and scope of the present invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

The invention claimed is:
 1. A driving method for an electrophoreticdisplay comprising a first surface on the viewing side, a second surfaceon the non-viewing side and an electrophoretic fluid which comprises afirst type of particles, a second type of particles and a third type ofparticles, all of which are dispersed in a liquid, wherein a) the threetypes of pigment particles have optical characteristics differing fromone another; b) the first type of pigment particles and the second typeof pigment particles carry opposite charge polarities; and c) the thirdtype of pigment particles has the same charge polarity as the secondtype of pigment particles but a lower zeta potential, the methodcomprising the following steps: (i) applying a first driving voltage toa pixel in the electrophoretic display for a first period of time, thefirst driving voltage having a polarity driving the first type ofpigment particles towards the first surface, thereby causing the pixelto display the optical characteristic of the first type of pigmentparticles at the first surface; (ii) applying a second driving voltageto the pixel for a second period of time, the second driving voltagehaving a polarity driving the third type of pigment particles towardsthe first surface, thereby driving the pixel towards the opticalcharacteristic of the third type of pigment particles at the firstsurface; and (iii) applying a third driving voltage for a third periodof time, the third driving voltage having the same polarity and the samemagnitude as the first driving voltage, and the duration of the thirdperiod of time being from about 10 to about 60 percent of the durationof the first period of time, thereby producing a mixture of the opticalcharacteristics of the first and third types of particles at the viewingsurface.
 2. The method of claim 1, wherein the duration of the thirdperiod of time is from about 20 to about 40 percent of the duration ofthe first period of time.
 3. The method of claim 1, further comprisingapplying a shaking waveform prior to step (i).
 4. The method of claim 3,further comprising applying, prior to the shaking waveform, a drivingvoltage driving the first type of pigment particles towards the firstsurface.